The present invention relates generally to polycrystalline gallium nitride (GaN) and more particularly to sintered polycrystalline GaN and its production by high pressure/high temperature (HP/HT) techniques.
Polycrystalline GaN is useful in a number of applications, primarily motivated by the rapid growth of GaN-based optoelectronics and electronic devices. These applications include, inter alia, sputtering targets and source material for bulk crystal growth.
Two methods for producing polycrystalline GaN are reported in the art. In the first method, GaN powder is cold pressed into a compact. Balkas, et al., J. Cryst. Growth, 208, 100 (2000). Unfortunately, little or no chemical bonding between GaN grains is present in the cold pressed material, as the apparent density is only about 2 g/cm3, much lower than the theoretical density (6.1 g/cm3). It is of marginal utility as a sputter target, due to residual porosity and moisture sensitivity. It also is not useful as a source for crystal growth, because it rapidly disintegrates back into powder in the presence of gallium nitride solvent.
In the second method, chemical vapor deposition (CVD) is used to form polycrystalline GaN films. Several proposals exist on this second method. For example, U.S. Pat. No. 6,113,985 proposes a method whereby ammonium chloride transports Ga atoms from a Ga metal source and deposits them as polycrystalline GaN on a substrate. Argoita, et al., (Appl. Phys. Lett., 70, 179 (1997)) teach a method for treating Ga metal in a nitrogen-containing plasma, thereby forming a polycrystalline GaN film on the surface of the Ga metal. However, these CVD-like methods are expensive and do not generate a thick, dense GaN part. In addition, the grains in CVD-grown GaN are large and columnar, reducing the strength and fracture toughness. Finally, the surface of CVD-grown polycrystalline GaN films is rough, which is undesirable for use as a sputter target.
Thus, there exists a need in the art to be able to fabricate polycrystalline GaN parts of sufficient density, etc., that they are suitable for a variety of commercial uses.